In recent years, differences in various material properties have been becoming revealed, with a particle diameter of 100 nm as a boundary. For example, pigments are formed such that the particle diameters are at least a half of the wavelength of visible light for the sake of high coloring and concealing abilities. However, when the particle diameters are 100 nm or smaller, they do not show chromatic properties. Thus, the development of new toning has been expected. Meanwhile, magnetic materials show a phenomenon that multiple magnetic domain particles become single magnetic domain particles and magnetic properties change at a boundary of around 100 nm. In expectation of such a novel property, studies have been made actively on ceramic materials, colored pigments, optical materials, electroconductive materials, piezoelectric materials and the like. Meanwhile, as to the relationship between particle diameters of nano order (100 nm or smaller) and transparency, absorption, scattering and reflection must be low to improve transparency. Further, it is known that when particle diameters are equal to or smaller than ¼ of the wavelength of visible light, scattering is low and transparency is high (refer to New Development of Nano Fine Particles, Toray Research Center, Inc., pp. 3 to 6).
In general, as exemplified by titanium white powders of fine particles, a transparent cured product is still obtained by adding inorganic oxide particles powders having a refractive index different from that of a polymerizable compound by 0.1 or higher, an X-ray contrast property and a primary particle diameter of nano order in an amount of at least about 3 parts by weight to 100 parts by weight of the polymerizable compound, adding a polymerization initiator and then fully kneading them to polymerize them. The reason is as follows. That is, since the surface energy of primary particles becomes higher and the primary particles become more unstable as the inorganic oxide particles become finer, the primary particles are agglomerated firmly to lower the surface energy, thereby becoming stable. That is, when the particles are added to the polymerizable compound, the size of the agglomerated particles is already nearly equal to or larger than the wavelength of visible light, and it is extremely difficult to re-disperse most of the agglomerated primary particles, so that a transparent cured product cannot be obtained.
Meanwhile, (meth)acrylate-based materials are widely used not only as industrial materials but also as dental materials due to their properties such as excellent transparency and good surface gloss. However, a (meth)acrylate alone shows a high polymerization shrinkage ratio, and a cured product thereof shows poor mechanical strength and abrasion resistance. Thus, a number of studies have been made on a composite of the (meth)acrylate and an inorganic oxide. JP-A 60-11505 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)discloses a dental composition with improved abrasion resistance which is obtained by adding a spherical inorganic oxide having a specific particle diameter to a vinyl compound and then polymerizing them. However, since the particle diameter of the inorganic oxide used in this method is 100 nm or larger, it is difficult to say that the dental composition has excellent surface gloss, and the inorganic oxide may protrude or come off by abrasion, and the resulting abraded surface is a rough resin surface. Thus, when the composition is used for dental applications, it may cause such a problem as abrasion of the opposing tooth. Further, when a large quantity of an inorganic oxide having a particle diameter of 100 nm or smaller is added to a polymerizable compound, a significant increase in the viscosity of paste occurs due to the large specific surface area of the inorganic oxide and the paste becomes unusable. On the other hand, when the quantity of the inorganic oxide is decreased, the paste becomes sticky and sticks to a spatula, so that in order to be practically used, it has a problem to be solved with respect to moldability. Meanwhile, JP-A 5-209027 discloses that a composite composition having excellent transparency and rigidity is obtained by uniformly dispersing silica in a vinyl compound by use of colloidal silica and a silane compound. According to this method, since silica of 100 nm or smaller can be dispersed uniformly, the composition has excellent transparency, and it is free of a problem of moldability caused by an increase in viscosity. Further, JP-A 7-291817 discloses use of the above composite composition as a dental composition. However, since the composition uses colloidal silica, the inorganic oxide is limited to silica, and the obtained cured product has almost no X-ray contrast property. Therefore, when it is used as dental cement, a dental filler or the like, it can hardly be detected by an X-ray, so that it is difficult to determine whether a treatment is successful and a problem may occur. Further, the publication describes nothing about a method of uniformly dispersing an inorganic oxide other than colloidal silica in the polymerizable compound. In addition, when the composition is used in industrial applications other than dental applications, hardness and abrasion resistance are improved; however, since silica is not capable of absorbing high-energy light having a shorter wavelength than that of visible light, e.g., ultraviolet light or a radiation, a cured product thereof may be degraded by yellowing or cracking when the cured product is exposed to outside.